Reactive Programming with Swift 4

Differentiating between single-tuple and multiple-argument function types

f: (Void) -> ()

When using f: (Void) -> () for the type of a function argument, it is generally meant to be f: () -> (), so the migrator will suggest that you use this type instead. Otherwise, with the new rules in SE-0110 for Swift 4, you will need to call the f function as f(()).

To read more about the Swift proposals, you can visit https://Apple.github.io/swift-evolution/.